There is known a scheme for calculating the location of an unknown radio source by disposing receiving sensors that receive radio waves emitted from the unknown radio source, at a plurality of locations, and for measuring time differences of arrival of the radio waves received by the plurality of receiving sensors.
This positioning scheme is called TDOA (Time Difference Of Arrival) positioning, and is applicable not only to radio waves but also to waves such as acoustic waves and light.
In addition, for application fields, the positioning scheme can be applied to various applications such as not only outdoor positioning (e.g., urban areas, city areas, mountainous regions, valleys, seas) but also indoor positioning (e.g., houses, factories, malls, underground shopping malls, hospitals). In addition, the positioning scheme is also applicable to positioning of spacecraft in the space field, and radio source positioning that uses a spacecraft or the like as a receiving sensor.
Conventional TDOA positioning will be described.
FIG. 20 is an illustrative diagram showing an overview of TDOA positioning disclosed in the following Non-Patent Literature 1.
In the example of FIG. 20, three receiving sensors Rx1, Rx2, and Rx3 receive radio waves emitted from an unknown radio source. Since distances from the unknown radio source to the three receiving sensors Rx1, Rx2, and Rx3 differ from each other, the radio waves emitted from the unknown radio source reach the receiving sensors Rx1, Rx2, and Rx3 after the passage of periods of time that are determined according to the distances to the three receiving sensors Rx1, Rx2, and Rx3.
Hence, for example, by calculating the cross-correlation CCF (x1(t), x2(t)) between a received signal x1(t) of the receiving sensor Rx1 and a received signal x2(t) of the receiving sensor Rx2, a TDOA12 which is a time difference of arrival between the receiving sensor Rx1 and the receiving sensor Rx2 can be obtained based on the cross-correlation CCF (x1(t), x2(t)).
In this regard, t represents discrete time where AD (Analog to Digital) sampling is performed. Therefore, the received signals x1(t) and x2(t) are AD-sampled discrete-time signals.
Likewise, by calculating the cross-correlation CCF (x3(t), x1(t)) between a received signal x3(t) of the receiving sensor Rx3 and the received signal x1(t) of the receiving sensor Rx1, the value TDOA31 which is a time difference of arrival between the receiving sensor Rx3 and the receiving sensor Rx1 can be obtained.
If the value TDOA12 which is a time difference of arrival between the receiving sensor Rx1 and the receiving sensor Rx2 and the value TDOA31 which is a time difference of arrival between the receiving sensor Rx3 and the receiving sensor Rx1 can be obtained in the above-described manner, then as shown in FIG. 20, by performing a publicly known positioning computation process that uses the two values TDOA12 and TDOA31, the location of the unknown radio source can be calculated.
Although in the example of FIG. 20 the radio waves emitted from the unknown radio source are received as direct waves by the three receiving sensors Rx1, Rx2, and Rx3, radio waves emitted from the unknown radio source may be reflected by buildings and the like, and then reach the receiving sensors. Such radio waves are called multipath waves.
Since the TDOA positioning disclosed in the following Non-Patent Literature 1 does not assume the reception of multipath waves by the three receiving sensors Rx1, Rx2, and Rx3, accuracy in calculating the location of the radio source degrades under an environment where multipath waves are received.
The following Non-Patent Literature 2 discloses a positioning device that calculates the location of a radio source with high accuracy even under an environment where multipath waves are received.
In this positioning device, when TDOAs resulting from a multipath wave are obtained in addition to TDOAs resulting from direct waves, the unnecessary TDOAs resulting from a multipath wave are eliminated using the measurement values of received signal strength (RSS), and the location of an unknown radio source is calculated based the remaining TDOAs resulting from direct waves.
Note, however, that in the positioning device disclosed in the following Non-Patent Literature 2, it is premised that there is one unknown radio source and the number of arrival waves which are direct waves is one. Hence, under an environment where there are two or more unknown radio sources, unnecessary TDOAs resulting from a multipath wave cannot be eliminated in principle.
FIG. 21 is an illustrative diagram showing an example of an environment where direct waves and multipath waves that are emitted from two radio sources (1) and (2) interfere with each other.
FIG. 22 is an illustrative diagram showing the number of TDOAs obtained by cross-correlation computation between received signals of receiving sensors of FIG. 21.
FIG. 23 is an illustrative diagram showing an example of the cross-correlation CCF(x1(t), x2(t)) between a received signal x1(t) of a receiving sensor Rx1 and a received signal x2(t) of a receiving sensor Rx2.
In the example of FIG. 21, the receiving sensor Rx2 receives a received signal x2(t) where a direct wave and a multipath wave that are emitted from the radio source (1) interfere with each other. In addition, a receiving sensor Rx3 receives a received signal x3(t) where a direct wave and a multipath wave that are emitted from the radio source (2) interfere with each other.
As a result, when the cross-correlation CCF(x1(t), x2(t)) between the received signal x1(t) of the receiving sensor Rx1 and the received signal x2(t) of the receiving sensor Rx2 is calculated, as shown in FIG. 23, three correlation peaks (the three values TDOA12,k (k=1, 2, and 3)) are obtained. The subscript “12” of the TDOAs indicates that the TDOAs are related to the receiving sensor Rx1 and receiving sensor Rx2, and “k” is the number assigned in turn to the TDOAs.
Likewise, when the cross-correlation CCF(x2(t), x2(t)) between the received signal x2(t) of the receiving sensor Rx2 and the received signal x3(t) of the receiving sensor Rx3 is calculated, four correlation peaks (the four values TDOA23,k (k=1, 2, 3, and 4)) are obtained, and when the cross-correlation CCF(x3(t), x1(t)) between the received signal x3(t) of the receiving sensor Rx3 and the received signal x1(t) of the receiving sensor Rx1 is calculated, three correlation peaks (the three values TDOA31,k (k=1, 2, and 3)) are obtained.
In the example of FIG. 21, despite the fact that the number of the radio sources (1) and (2) is two, a larger number of TDOAs than the number of radio sources are obtained.
Namely, since TDOAs resulting from a multipath wave are obtained in addition to TDOAs resulting from direct waves, a larger number of TDOAs than the number of radio sources are obtained.
If positioning computation for the radio sources (1) and (2) is erroneously performed using unnecessary TDOAs resulting from a multipath wave, positioning cannot be performed properly, and thus, there is a need to eliminate the unnecessary TDOAs resulting from a multipath wave. However, as described above, the positioning device disclosed in Non-Patent Literature 2 cannot eliminate, in principle, unnecessary TDOAs resulting from a multipath wave under an environment where there two or more unknown radio sources.